That art was worth taking as a subject
That your grades and comments on behavior go on your “permanent record”, strongly implying that those things follow you your whole life.
That we were moving swiftly toward adopting the metric system.
That penmanship was important.
That Catherine the Great of Russia died by being crushed to death under a horse while attempting to have sex with it.
My teacher was a very strange person. 
French was the international language.
Penguins are mammals because they don’t fly, whales are fish because they swim.
That’s not a lie, it’s an opinion.
That all the accused “witches” in Salem were misunderstood teenage girls. No, the girls were doing the accusing.
-
That robins have red breasts ecause their blood shows through their thin skin
-
that mosquitos give you malaria by laying their eggs in your bloodstream.
-
that airplanes fly because they “float on a cushion of air”
Actually, I don’t think these were technically “lies”, because I’m pretty sure my teachers believed these things.
The entire myth of the American marksmen skirmishing in the woods while the Redcoats stood in line like dumbasses. News flash: The British knew what skirmishing was, and those savvy American guerillas usually lost. It was a profoundly anachronistic understanding of what 18th century warfare was about.
That Jesus was hung from a pole. Not a cross. A pole. Sixth grade PUBLIC school.
They tossed her out about a month later. Thanks Mom!
It’s only at the age of 40 that I learned that it is not, in fact, illegal to kill a preying mantis.
Not direct lies so much as anecdotes as truth - years after I was out and the Internet came along, Snopes blew away several things I was told. Some I still recall:
[ul]The Chevy Nova didn’t sell in Mexico because it means “it doesn’t go”
[li]Catfish the size of cars live at the bottom of the dam at <local> lake[/li][li]Guy gets flat outside Insane Asylum, inmate suggests using one lug nut from the other tires[/li][li]<local> College library is sinking because the architect forgot to take into account the weight of the books[/li][li]Golf means “Gentlemen Only, Ladies Forbidden”[/li][li]You can’t swim for an hour after eating[/li]Swallowing gum takes 7 years to digest[/ul]
“I’m sure that the guilt you feel is far worse than any punishment you might receive. Now, don’t you feel terrible?”
5th grade SCIENCE teacher said sound was faster than light.
A driving school instructor once told me that it was perfectly legal to skip a red light by taking a shortcut through a gas station on the corner. Years later, a different instructor said that such an action was NOT legal. I still don’t know what the truth is.
“When you get to high school, teachers are going to start making you write in cursive!”
That Old Faithful geyser erupts every hour, on the hour, like God intended. Years later, I listened to a ranger talk at Yellowstone where they said they never post predicted times exactly on the hour precisely to dispel that myth.
In many places(every one I have lived in) it is illegal, usually under some vague language like “Improper travel to avoid a traffic control device or officer”